The present invention relates to organic aerogels, particularly to aerogel microspheres, and more particularly to resorcinol-formaldehyde, carbon, and melamine-formaldehyde microspheres and a method of fabrication which involves inverse emulsion polymerization, wherein the size and structure of the microspheres is determined by the chemical formulation and processing parameters.
Aerogels are a special class of open-cell foams with unique thermal, acoustic, optical, and electrical properties. Foamed organic polymers and organic foam composite materials are known and used in the insulation, construction, and similar industries. Also, machinable and structurally stable low density, microcellular carbon foams and catalyst-impregnated carbon foams are known. In addition, electrically conductive, open-celled, low density, micro-cellular carbon foam has been developed. Development efforts have also been directed to low density aerogels which may be carbonized to form low density carbon foams with a cell size of &lt;0.1 micron for use in high-energy physics applications, waste management, such as adsorption of fluids and toxic gases, ion exchangers, and supports for metal catalysts, etc., as exemplified by U.S. Pat. Nos. 4,873,218 issued Oct. 10, 1989; No. 4,997,804 issued Mar. 5, 1991; and No. 5,086,085 issued Feb. 4, 1992, each to R. W. Pekala, and paper UCRL-99846, "Resorcinol-Formaldehyde Aerogels And Their Carbonized Derivatives", by R. W. Pekala et al., Oct. 24, 1988. High density, micro-cellular carbon foams of this type have also been developed for use, for example, in electrochemical double layer supercapacitor applications, as described and claimed in copending U.S. Application Ser. No. 07/822,438 filed Jan. 17, 1992, entitled "Supercapacitors Based On Carbon Foams" now U.S. Pat. No. 5,260,855, issued Nov. 9, 1993. Recently, high density carbon aerogels are being developed in a much wider variety of applications, such as in energy storage and energy conversion devices, adsorption/filtration media, electrochemical double-layer desalination, dilute solution metal recovery, hazardous waste treatment, and chromatographic packings.
Thus, while there has been substantial effort directed toward the development of foams and aerogels for use in energy storage devices (capacitors, rechargeable batteries, fuel cells), hazardous waste water treatment, catalyst supports, and insulation and construction applications, the processes for producing these materials have resulted in the formation of large monolithic pieces, slabs or chunks. More recent efforts have been directed to forming organic aerogels and then chopping or grinding the aerogel to produce particles which are mixed with a binder to produce a composite, exemplified by U.S. Application Ser. No. 08/057,739, filed May 4, 1993, and entitled "An Aquagel Electrode Separator For Use In Batteries And Supercapacitors". Also, it has been recently discovered that the aerogels can be doped with various dopants during the fabrication process, as described and claimed in copending U.S. Application Ser. No. 08/041,507, filed Apr. 1, 1993, and entitled "Doping Of Carbon Foams For Use In Energy Storage Devices". It has been recognized that the formation of organic aerogel foam in small spheres (microspheres) would greatly expand the use of these materials, as well as reduce the process time and equipment costs in applications where they would be appropriate. For example, for energy storage applications, the microspheres have the advantage over slabs of aerogels by allowing greater mechanical flexibility of a composite electrode, or may, using an appropriate binder be used in a single cell, low voltage "jelly roll" cell, similar to conventional "AA", "C" or "D" cells, but with lighter weight. Aerogel microspheres are also useful in packed bed reactors (chromatographic packings, deionization processes, etc.) in that connective flow through the bed can occur with relative ease (when compared to flowing fluid through a monolithic slab), while maintaining a very high surface area packing. In addition, the aerogel micropheres may be utilized in air filtration and medical applications, as well as in energy storage applications, such as in a double-layer capacitor.
The present invention provides a method for producing microspheres of organic aerogel foam, and thus advances this field of technology by enabling such foam to be used in various applications not suitable or cost effective for aerogel monoliths.
As the result of further research and development, it has been recognized that organic aerogel microspheres can be produced in a wide range of spherical diameters, densities, and surface areas, by controlling the chemical formulation and processing conditions and procedures wherein the aqueous solution is stirred in a material in which the aerogel reactants and products are insoluble, such as mineral oil or cyclohexane, during the polymerization/gelation phase of the process. Thus, the present invention improves on the state of the organic aerogel art by providing control over the size of the aerogel microspheres, either doped or undoped in the organic or carbonized state, and a method for forming the microspheres.